This study is an open-label, randomized crossover study to compare the pharmacokinetics of sulfadiazine given on a twice-daily basis with a four-times-daily schedule. Ten HIV-infected patients with CD4 counts below 500 will be enrolled. Patients must not have concurrent opportunistic infections and must be willing to comply with the study regimen. The study is designed to compare pharmacokinetic parameters and serum concentrations of sulfadiazine at two doses: 1000mg QID and 2000mg BID. Additionally, this study will assess the safety and tolerability of these dosage regimens. Currently, the study has received Institute Review Board approval and will begin enrolling within the next 1 to 2 months.